A national investigation led by Mayor Bloomberg uncovered some pretty shocking numbers. Between February and May of this year, more than 25,000 thousand guns were sold illegally on the internet. And no background checks required, giving people with criminal records a way to get around federal laws.

New York Mayor Michael Bloomberg, one of the most outspoken critics of the free-flowing sale of firearms in the United States, jumped back into the gun-control fray Wednesday, announcing the results of a study into the online marketplace for guns.

The study, sponsored by Mayors Against Illegal Guns, a coalition of more than 1,000 mayors advocating tougher curbs on firearm sales, looked into the online transactions of private gun sellers, who are not required by federal law to conduct background checks. The coalition was founded by Mayor Bloomberg and Boston Mayor Tom Menino in 2006.

"In the digital age, convicted felons, domestic abusers, and other dangerous people who are legally barred from buying guns can do so online with little more than a phone number or e-mail address,” Bloomberg said at City Hall on Wednesday. "And as our investigation shows, thousands of criminals and other prohibited purchasers are doing just that. In fact, not only are criminals buying guns online, they're doing so brazenly by openly advertising that they want to buy them.”

The mayor announced the findings two days after Navy reservist Aaron Alexis stepped into the US Naval Sea Systems Command headquarters in Washington, D.C., about a mile from the US Capitol, and gunned down 12 workers and wounded eight others before being killed by police.

The announcement also comes a week after Bloomberg suffered a stinging personal defeat when two state senators in Colorado, supporters of stricter gun control, lost their seats in the state’s first ever recall election.

The New York mayor had poured $350,000 into last Tuesday’s vote, which ousted Colorado Senate President John Morse and Sen. Angela Giron, both Democrats, who voted in favor of legislation requiring background checks for all firearm purchases and banning magazines with more than 15 rounds of ammunition.

In Wednesday’s remarks, the mayor singled out an online gun market called Armslist – a pun of the popular classified site Craigslist. The study only surveyed buyers on the site – a total of 13,000 want-to-buy listings, whose contact information was then matched up with criminal records – and found that 1 of 30 would-be buyers had felony or domestic abuse records that would not pass current background checks.

The study also notes that this figure does not include those prohibited from buying guns because of mental illness, drug abuse, or immigration status – which accounted for 25 percent of the sales blocked by the federal National Instant Criminal Background Check System in 2012.

"When you consider that Armslist will host some 800,000 unique firearm ads this year, including many that are completely anonymous, our investigation makes clear that we can safely estimate that more than 25,000 guns may be transferred to criminals each year through Armslist alone,” Bloomberg said.

The National Rifle Association, which also poured money into the Colorado recall election, poked fun at Bloomberg’s contributions, tweeting that he “wasted $350,000 on Colorado recall.”

The NRA also seized on comments made before the election to mock the mayor and his coalition. Senator Giron had said before her ouster, “For Mayors Against Illegal Guns, if they lose even one of these seats, they might as well fold it up. And they understand that.”

“Listening, Mayor Bloomberg? Colo. recall Senator said if she lost, Mayors Against Illegal Guns should fold up,” tweeted the nation’s largest gun lobby. The recall election was seen by many observers as another national bellwether for gun control.

But Monday’s rampage at the Navy Yard in the nation’s capital has renewed calls for the kind of gun-control measures supported by the Colorado senators who lost their seats last week.

On Tuesday, Senate Majority Leader Harry Reid said he would attempt to bring a vote on a bill to expand background checks for most gun purchases – a measure that flopped in April. Still, even though a number of Democratic and Republican senators – including Republican John McCain of Arizona – said Congress should act in the wake of the Navy Yard massacre, few see this effort succeeding.

"We're going to move this up as quickly as we can, but we've got to have the votes first," Senator Reid said Tuesday. "We don't have the votes. I hope to get them, but we don't have them."

Bloomberg, who is closing out 12 years in office as mayor of New York, called for Congress to act on bipartisan bills before both chambers of Congress. These would close the loophole in federal law that allows private parties to sell guns without background checks.

"[Federal] law should require a background check for every gun sale – not just those involving licensed dealers,” Bloomberg said. “In more than 40 states, criminals and other prohibited purchasers can avoid background checks by buying handguns or long guns from unlicensed 'private sellers' – often at gun shows or through anonymous online transactions.”